How to do it…

In the following steps, you will learn how to run instrumentation tests:

1. Let's create a simple app, which will just have the Hello World! text and a button:

2. On clicking the button, the text will change to Goodbye World!:

3. Now, let's write a test to verify this behavior.

In this recipe, we will learn how to run the espresso test; in the next recipe, we will learn how to write an espresso test, so just bear with me until I explain how to write an espresso test in the later recipe because it is complicated and needs an entire recipe to do justice to it.

Here's an espresso test:

class MainActivityTest {
    @Rule
    @JvmField var activityRule: ActivityTestRule<MainActivity> = ActivityTestRule(MainActivity::class.java)

    @Test
    fun testButtonBehaviour() {
        onView(withText("Hello World!"))
                .check(matches(isDisplayed()))
        onView(withId(button)).perform(click())
        onView(withText("Goodbye World!"))
                .check(matches(isDisplayed()))

    }

}

In the first line of the testButtonBehaviour method, we are checking whether Hello World! is appearing on screen. Then, we are performing a click operation on the button and finally checking whether Goodbye World! is appearing on the screen.

4. To run the preceding test, just right-click on the test class and select Run MainActivityTest:

5. Once you select that option, you'll be shown a dialog box that will ask which device you want to run the test on. You can either choose a real device or an emulator.
6. After that, you can see your tests running on your device (you will see the steps written in the code being performed on the device).